This invention generally relates to optical communications, and in particular to a method and system for aligning data signals transmitted on an optical carrier medium.
The backbone of point-to-point information transmission networks is a system of optically amplified dense wavelength division multiplex (DWDM) optical links. DWDM optical fiber transmission systems operating at channel rates of 40 Gb/s and higher are highly desirable because they potentially have greater fiber capacity and also have lower cost per transmitted bit compared to lower channel rate systems.
The modulation format of 40 Gb/s DWDM transmission systems is typically chosen to have high Optical Signal-to-Noise Ratio (OSNR) sensitivity. High OSNR sensitivity means that a low OSNR is sufficient to maintain a desired bit error ratio (BER) of the transmission or, equivalently, that the system is able to operate at a desired BER even in the presence of a high level of optical noise. In addition, modulation formats of 40 Gb/s DWDM transmission systems are typically chosen to be tolerant to optical filtering because existing systems sometimes include optical multiplexers and demultiplexers for 50 GHz channels spacing that limit the bandwidth. Also, existing systems sometimes include cascaded optical add-drop multiplexers.
Accordingly, Differential Phased Shift Keying (DPSK) has been considered for 40 Gb/s DWDM transmission systems, in part because DPSK transmission systems have excellent OSNR sensitivity. In a DPSK system, data is encoded onto a carrier wave by shifting the phase of the carrier wave. The amount of the phase shift may be selected based on the amount of data to be encoded with each phase shift. For example, in Differential Binary Phase Shift Keying (DBPSK), the phase of the signal may be shifted in increments of 180° (i.e., by π radians) in order to encode a single bit of data (“1” or “0”) with each phase shift. In Differential Quadrature Phase Shift Keying (DQPSK), the phase of the signal may be shifted in increments of 90° (i.e., by π/2 radians) in order to encode two bits of data (e.g., “11” or “01”) with each phase shift.
The number of possible phase shifts is typically referred to as the number of “constellation points” of a modulation format. For example, DBPSK has two constellation points, and DQPSK has four constellation points. Modulation formats using a different number of constellation points (e.g., “m” constellation points) are also known, and are referred to generically as DmPSK formats.
If both the phase of the signal and the amplitude of the signal are used to encode the signal with the data, then the modulation format is called QAM (quadrature amplitude modulation) or m-QAM, where m is the number of constellation points.
A shift in the phase of the signal is referred to as transmitting a “symbol,” and the rate at which each symbol is transmitted is referred to as the “symbol rate.” As noted above, multiple bits of data may be encoded with each symbol. The rate at which bits are transmitted is referred to as the “bit rate.” Thus, the symbol rate in a DQPSK system is half the bit rate. For example, a DBPSK system and a DQPSK each transmitting at the same symbol rate would evidence different bit rates—the DQPSK system would have a bit rate that is twice the bit rate of the DBPSK system.
Accordingly, a 43 Gb/s data rate in a DQPSK system corresponds to 21.5 Giga symbols per second. Thus, DQPSK transmission systems have a narrower spectral bandwidth, greater chromatic dispersion tolerance and greater tolerance with respect to polarization mode dispersion (PMD) compared to traditional formats and compared to DBPSK.
DBPSK and DQPSK can be of the non-return-to-zero (NRZ)-type or, if a return-to zero (RZ) pulse carver is added to the transmitter, may be of the RZ-type.
Generally, a DmPSK signal may embody one or more data signals. For example, two or more modulators may each receive an electrical signal representing data digitally and electrically encoded as individual bits. The modulators may modulate an optical carrier to encode each data signal, and the resultant outputs may combine to encode multiple data streams onto a DmPSK optical signal.
If multiple electrical data signals are encoded onto an optical carrier, the electrical signals need to be aligned with each other. This is because the signals are eventually added together to form the optical signal. If the electrical signals are aligned to each other, then the full range of the optical amplitude containing the encoded data in its phase is used to encode the transmitted signal. However, if the electrical data signals are not aligned to each other, then the peaks of the data signals will not line up, and as a result the phases of each optical signal are not added correctly to give the correct transmitted phase.
For example, when two data signals are both at a peak (e.g., approximately 100% power, although 100% power is not required) and are added together, the result is that the combined output optical signal is at a maximum optical power. When two data signals are both misaligned, then there is a reduction in transmitted power.
However, if the electrical data signals are not properly aligned in time, then peaks of the signals do not properly line up. For example, consider what occurs if the first data signal and the second data signal are both at a peak, but are not properly aligned so that the second data signal reaches the peak slightly after the first data signal. In this case, the first data signal may be at a peak (100% power) when the data is encoded onto the optical carrier, while the second data signal may not have yet arrived at the peak (e.g., the second data signal may be at 80% power but approaching 100%). In this situation, the output optical signal may be at only 90% power.
As a result, the multiple signals need to be aligned to each other at the transmitter so that the output power of the optical signal is properly utilized and that the phase of each optical signal is correctly added, and so that the resulting signal can be properly interpreted at the receiver. Conventionally, signal alignment is done by manually reviewing a representation of a first signal and a second signal with an oscilloscope, and manually establishing static phase adjustment values that allow the signals to be aligned. The phase adjustment values are typically used for the life of the transmitter unless they are manually changed.
Problematically, manually establishing static phase adjustment values requires significant amounts of time and is an imprecise procedure. Further, the static phase adjustment values need to be recalculated if changes are made to the transmitter, which may necessitate taking the transmitter offline while the phase alignments are recalibrated. That is, because the values are not dynamic, they cannot adapt to changes in the system over time. This can cause issues because correct phase alignment cannot be guaranteed over the life of a signal and at different bit rates—if errors are present in the adjustment values due to a change in a characteristic of the signal or transmission line, or due to improper establishment of the adjustment values, the error cannot be corrected without manually recalculating and reconfiguring the static phase adjustment values.